Drive mechanisms including actuators are conventionally utilized to control the selective positioning of one or more members of a system. System design requirements dictate, and often limit, the specific design suitable for the application. In some environments, unstable actuators are utilized, such as but not limited to bi-stable actuators.
Rotary solenoid actuators advantageously provide high starting torque, but continually accelerate along the length of travel, resulting in impact at the end of travel. Feedback loops are typically not used with rotary solenoids, so the velocity at the end of travel is typically much higher than needed. Adding position feedback sensors defeats the packaging advantages of using the solenoid.
The problem becomes more pronounced when the actuator is bi-stable, with significant detent forces at the ends of travel holding the actuator into the stops. While this arrangement is highly desirable from a power consumption perspective, it requires higher torque application at the beginning of travel, resulting in high velocities at the end of travel.
Prior applications have tried to limit the impact velocity by limiting the duration that the actuator is powered on. These methods include applying current for only a short duration (timing), and shutting the actuator off after tripping a proximity sensor. These methods work better on statically neutral actuators, where the actuator will coast freely after power is removed. These methods require estimating the amount of time and torque required to meet the travel requirements, particularly when parameters are variable over the operating environment. Furthermore, timing is only partially effective with bi-stable actuators, since the actuator will freely accelerate once it is past the detent position, resulting in high velocity at the end of travel. Furthermore, removing power too soon will result in the actuator not overcoming the detent torque and fail to move to the alternate position as commanded.
What is desired is a means of providing an bi-stable actuator having a high starting torque, but a slower, regulated velocity as the actuator moves through its range of travel, without the added weight and volume of a position feedback transducer.